1. Field of the Invention
This invention relates to a novel polyimide which is soluble in organic solvents and excels in heat resistance, and to a process for producing the same.
2. Related Art
Generally, polyamide resins excel in heat resistance, but many of them have poor processing abilities due to their insolubility and infusibility. For this reason, in order to form a polyimide into a film, etc., a process for obtaining a polyamide mold by using a varnish with a polyamic acid, which is a precursor of the polyimides, dissolved in an organic solvent for molding and processing, and carrying out imidation through a dehydration ring closure reaction. However, since the polyamic acid varnish easily undergoes hydrolysis and amide interchange reaction, it is necessary for preventing the polyamic acid varnish from being changed into a low molecular weight form and from gelation to store it at a low temperature. Furthermore, since the solvent used easily absorbs moisture, there is a problem that the moistened resin is separated from the varnish. Moreover, there is also a problem that after the formation of the film, etc., the condensed water formed during the imidation reaction forms voids in the products. In addition, since a high temperature of not less than 300.degree. C. is required for carrying out the imidation reaction, it is very difficult to provide a polyimide coating layer on a base having no heat resistance.
Consequently, in order to solve the problems associated with the polyimide, studies and developments have been made to obtain polyimides capable of forming and processing by selecting a raw material having solubility in an organic solvent or having a softening point, and many suggestions have been made. For example, Japanese Patent Laid-Open 10051/1987 discloses a polyimide obtainable from a diamine represented by the following formula (12) and pyromellitic acid. However, this polyimide has a softening point of not less than 300.degree. C. and has forming and processing ability, but since it is bad soluble in an organic solvent, a varnish of polyimide, which is a precursor, must be used for forming a film, etc. The problems described above have yet been left. ##STR2## wherein X" is --SO.sub.2 -- or --C(.dbd.O)--.
Japanese Patent Laid-Open 263116/1988 and Japanese Patent Laid-Open 263117/1988 disclose polyimides obtainable from 4,4'-methylenebis(2,6-dialkylaniline) and a tetracarboxylic dianhydride such as biphenyltetracarboxylic dianhydride or pyromellitic dianhydride. These polyimides are soluble in N-methyl-2-pyrrolidone and m-cresol, and have a glass transition temperature of not less than 400.degree. C. However, these solvents which have high boiling points are not suitable for forming films, etc.
Japanese Patent Publication 18914/1968 discloses a polyimide obtainable from p-phenylenebis(trimellitate) dianhydride and a diamine such as benzidine and 4,4'-diaminodiphenyl ether, etc., and Japanese Patent Publication 5911/1968 discloses that bisphenol A bistrimellitate dianhydride may be used as raw materials for a polyimide. However, the polyimides obtainable by these processes have not yet been reported to be dissolved in a low polar solvent.
Japanese patent Laid-Open 258225/1985 discloses a polyimide obtainable from 1,4-bis(p-aminocumyl)benzene and an aromatic acid dianydride or aliphatic acid dianhydride. The polyimide has merits that it is soluble in N-methyl-2-pyrrolidone and its glass transition point is lower than 200.degree. C., but has demerits that it is insoluble in a low polar solvent and a low boiling-point solvent and its heat resistance is low as for the mechanical strength. Japanese patent Laid-Open 11633/90 discloses a process for producing a polyimide from a diamine as represented by the above formula (12) and bisphenol A bistrimellitate dianhydride. The polyimide is not only soluble in N-methyl-2-pyrrolidone but also in N,N-dimethylformamide, dioxane, etc., but is insoluble in diethylene glycol dimethyl ether. Furthermore, the diamine described above is difficult to be produced, which indicate the difficulty of industrially carrying out this process for producing a polyimide.
Japanese Patent Laid-Open 78481/93 discloses a polyimide obtained from a diamine represented by formula (8) and bisphenol A bistrimellitate dianhydride or a dianhydride represented by the following formula (13). The polyimide is not only soluble in N-methyl-2-pyrrolidone and N,N-dimethylformamide, but also in dioxane, tetrahydrofuran, diethylene glycol dimethyl ether, toluene, etc. However, since the acid dianhydride which is the raw material is difficult to be produced in high purity, the process is proven to be inadequate for industrial production. No polyimide having sufficient flexibility is obtainable from a diamine represented by the formula (8) (in this patent, R.sup.1 .dbd.R.sup.2 .dbd.R.sup.3 .dbd.R.sup.4 .dbd.isopropyl) and bisphenol A bistrimellitate dianhydride. ##STR3##
In light of such situations, the present invention has been done to make an improvement. An object of the present invention is, therefore, to provide a novel polyimide which is soluble in various organic solvents and which excels in heat resistance and a process for producing the same.